1. Technical Field
This invention pertains to a transmission which provides continuously variable gear ratios. Within a finite range, the transmission will automatically provide the optimum gear ratio to match the driving force to the load. The transmission is suitable for general use, in applications including automobiles, bicycles, and various machines.
2. Background Art
Standard vehicle and machine transmissions are restricted to a finite number of gear ratios. Recognizing the efficiency that would result from a continuously variable, or "stepless", transmission, a number of designs have been attempted. U.S. Pat. No. 3,722,305 to Walters et al. describes a drive mechanism particularly designed for a marine winch. A rotary cam having a plurality of lobes engages a plurality of cam followers as the cam is rotated. The cam followers, in turn, engage separate levers which are connected to over-running clutches.
Similarly, a variable mechanical transmission is described in U.S. Pat. No. 3,750,485 to Blakemore. A continuous mechanical drive is effected by means of a plurality of one-way clutches connected in tandem between a rotary output member and a plurality of variable excursion mechanical oscillators shifted in phase in relation to each other and driven by a rotary input member.
U.S. Pat. No. 4,475,412 to Evans presents a transmission wherein arm driven gear plates alternately engage and drive an output shaft. The gear components of this device are in positive engagement with one another, avoiding the drawbacks of pliable or belt power transmission components.
Although the devices described in the Walters, Blakemore, and Evans patents may constitute functioning continuously variable transmissions, these transmissions share certain disadvantages. The complexity of each of these prior designs and the large number of moving parts restricts them to low speed applications. In short, these transmissions are not practical for automobiles or other fast moving vehicles or machines.
The transmission device described in U.S. Pat. No. 4,326,431 to Stephenson is comprised of a drive wheel and a driven wheel which are positioned adjacent and parallel to each other. Movement imparting bars of the drive wheel engage movement receiving pegs of the driven wheel. A range of gear ratios is obtained by varying the relative position of the two wheels. The Stephenson transmission provides a variable number of gear ratios within the limits of 1:1 and 2:1. In order for the movement imparting bars to mesh with the movement receiving pegs the distance between the two elements must be exact multiples of each other. This would only occur in a limited number of ratios; other ratios would necessitate a series of jumps in the speed of one of the wheels as they rotate, perhaps resulting in rough operation and stress on the component parts.
Automobile manufacturers have long realized the efficiency and advantages of a continuously variable transmission, and have attempted to achieve such a transmission through devices other than the patented transmissions described above. Thus, several automobile manufacturers appear to have focused on the transmission design described in Popular Science articles of March, 1984 (Page 48), and September, 1987, (Page 56). This transmission relies on a belt running on movable cone pulleys. Although numerous types of belts have been attempted, none seems to have a sufficient life to be practical, or sufficient strength to be used with heavy loads. Furthermore, hydraulic or electronic controls are needed to set the pulley ratios and control a clutch. The power required to run the hydraulic system also reduces efficiency. While continuously variable transmissions are known in the prior art, all of the existing continuously variable transmissions share the limitation of either not being suitable to heavy loads, such as in vehicles other than compact cars or not capable of handling high speed applications.